How it grew: In the past five years, Stericycle has acquired 142 companies, primarily in the domestic medical waste disposal sector, with 45 acquisitions in 2011 alone. As a result, revenue shot up more than 16 percent, to nearly $1.7 billion, in 2011, from about $1.4 billion in 2010. "It's a density game," CFO Frank ten Brink says. "More customers in a geographic area make us more effective." Now Stericycle is looking overseas, including a recent acquisition in Spain, and adding services domestically, to fuel growth. Last year, Stericycle entered the patient communication business, buying Franklin, Tenn.-based NotifyMD, which provides call center services to health care providers.